1. Field of the Invention
The subject invention relates generally to integrated circuits and more particularly to the provision of operative contacts in MOSFET devices.
2. Description of the Prior Art
Metal oxide semiconductor field-effect transistor (MOSFET) devices are characterized by a silicon substrate having a pair of diffusions separated by a channel over which a conductive gate extends. Conduction between the diffusions is controlled by application of a control voltage to the gate. Although initially the gate was formed of metal, more recently doped silicon has also been used for this purpose. The present invention is directed at both types of MOSFET devices, although it will probably be found particularly useful with silicon gate MOSFET devices.
In devices of the above type, contact between various portions of circuits in the substrate is established by means of conductors extending over the surface of the silicon, these conductors advantageously being formed of polycrystalline silicon in a manner well known to those skilled in the art. This is particularly convenient where the control gates are also formed of polycrystalline silicon.
In order to make the necessary interconnections, it is often advantageous to make a number of contacts between the polycrystalline silicon conductor and the substrate. This is done by forming doped (and hence relatively highly conductive) regions in the substrate surface, covering the surface with a masking oxide layer, creating openings in that layer so as to expose a portion of each conductive region, depositing a polycrystalline silicon layer so that it extends through the openings to establish electrical contact to the doped regions and patterning the layer into individual polycrystalline conductors. The type of contact thus formed is usually referred to as a "buried" contact, and will be so referred to herein.
During the processing steps which precede the deposition of the polycrystalline silicon layer, a thin oxide layer tends to form over the doped regions where they are exposed through the openings in the masking oxide layer. This does not create a problem where the interconnection which is to extend into contact with a doped region is made of aluminum, as is sometimes the case. This is so because the aluminum will be driven through such a thin oxide layer (on the order of 50 angstroms) during subsequent annealing steps. However, where the interconnection is made of polycrystalline silicon, which will usually be the material of choice for the interconnections when the control gates of the devices are themselves made of polycrystalline silicon, the intervening thin oxide layer may block conduction. As a result, when a wafer is fabricated with a large number of doped regions to which contact is to be made from a number of polycrystalline silicon conductors extending over the surface of the wafer, a significant number of defective contacts may be formed.
The problem just described, while usually occurring with wafers which are formed of silicon throughout, may also occur on wafers of other semiconductor materials, and where silicon or another semiconductor is deposited on an insulator such as sapphire or glass, with the MOSFET devices being formed in the thin semiconductor layer which is mechanically supported on the underlying insulating substrate.
It is the principal object of the present invention to overcome the problem of defective contacts in MOSFET devices formed in silicon wafers.
A closely related object of the invention is to provide a technique whereby defective contacts may be readily repaired, so as to render them fully operative.